Underground steel pipelines carrying gas up to 300 psi are routinely joined together with bolted couplings that have internal rubber seals. These seals are pushed against the surface of the pipe sections as the bolts are tightened making the gas tight seal on each pipe section. It is imperative that such high pressure piping and their connectors be free of leaks. High pressure pipe leaks are particularly dangerous, often times leading to explosions resulting in extensive loss of property and even serious or fatal injuries to bystanders. However, over the years pipe sections can pull out of their bolted couplings through ground movement or third party damage. This will cause the very catastrophic leaks which must be avoided.
Most attempts to address gas leaks from pipes are reactive. That is, the problem of leakage is only first addressed when the leak itself is recognized. This is generally not a problem in repairing leakage from low pressure (e.g. up to 2 psi) or medium pressure (e.g. up to 60 psi) piping. These low and medium pressure leaks are often repaired by the encapsulation of the leaking joint or coupling. In the encapsulation process, the leaking joint is first grit blasted down to bare metal and a primer is applied. A mold is placed around the joint and a synthetic resin sealant of relatively low tensile strength, i.e. approximately 500 psi, is mixed and poured into the mold and then pressurized to above the gas pressure in the pipe. This method stops the leak, as the gas cannot move from a low pressure to a higher pressure. The resin sealant cures within the next hour to a solid rubber type material that is permanently bonded to the pipe, yet remains flexible. Flexibility of the joint is important, in this situation, because these pipes are generally made from twelve foot lengths of cast iron, which is not ductile and would break if they were not allowed to flex at the joints. Tests have shown that such encapsulation methods have successfully maintained sealed pipe joints for a minimum of fifty years.
However, there are relatively few methods which are available for high pressure steel piping leak prevention; that is, methods which address strengthening pipe sections at their couplings so as to prevent the pipe sections pulling out of the couplings, causing a catastrophic leak. Past preventative maintenance methods include replacement of standard bolted couplings by reinforcing restraint couplings. These are similar to bolted couplings, but have added integral gripping rings positioned at either side of their rubber seals. As the bolts are tightened, these gripping rings are pushed down the slope of the coupling barrel and dig into the pipe sections preventing the pipes from pulling out of the coupling. Replacing a standard bolted coupling with a restraint coupling on an existing pipeline is highly impractical and very expensive.
Where a pipeline can be turned off, the gas must be stopped at either side of the coupling by using a high pressure, multiple stopple system. Disposable stopple fittings, which are expensive, must first be welded to the pipe. A high pressure drill is attached to the fittings and multiple large holes are drilled into the pipe. The drill is removed and a stopple is inserted through each hole and deployed to stop the flow of gas. One or two stopples are placed on both sides of the coupling, with sufficient room between them to cut out the section of pipe containing the coupling. There is normally insufficient gap between the ends of the two original pipes to allow removal of the coupling or install the new one. As a result, this piping section must be cut out and replaced with a spool length of pipe and two restraint couplings. The stopples are removed and the stopple fittings are capped. While these expensive fittings must remain on the pipe, they still may be a future source of leakage. Where the pipeline must always be in service, which is usually the case, the above method is used with the addition of a full pressure by-pass being installed to re-route the gas, while the short section of main is out of service. Both of these methods of switching out the couplings require substantial excavations and a large crew with safety personnel standing by. This, of course, is very expensive, time consuming, and a disruption to the operation.
Another common method of high pressure leak prevention consists of welding a split sleeve fitting or “pumpkin” over the coupling. This is also an expensive proposition. Apart from the cost of the pumpkin, this device must be welded by a certified high pressure pipeline welder, a specialized individual whose service creates even more expense. Significantly, there are also major safety concerns with welding onto in-service pipelines. Burn through will occur if the unmelted area beneath the weld pool can no longer contain the pressure within the pipe or equipment. This will cause the contents of the pipe to escape and hit the molten metal with disastrous results.
Another concern is for the integrity of the pipeline following welding, since welds made in-service cool at an accelerated rate, as the result of the ability of the gas flowing through the pipe to remove heat from the pipe wall. The pipe itself is also a heat sink and the outside temperature and wind can have an additional cooling effect. The welds, therefore, are likely to have hard heat-effected zones and a resulting susceptibility to hydrogen cracking. In addition, the metallurgy of the pumpkin and the weld materials must be compatible with the metallurgy of the pipe. Special welding consideration may be needed for high tensile strength steels to avoid weld cracking and there may be a need for post-weld heat treatment.
It is quite evident that existing methods for addressing the potential for pullout of the pipe sections from the couplings of high pressure piping are expensive and afford a number of significant disadvantages and risks.